Measurement architecture to obtain per-hop one-way packet loss and delay in multi-class service networks

ABSTRACT

An architecture for measurement of per-hop, one-way delay includes a input observation circuit ( 24 ) at the input interface of a node ( 12 ) and a output observation circuit ( 26 ) at the output interface ( 16 ) of a node ( 12 ). The input observation circuit ( 24 ) copies and time stamps the headers of incoming packets, and filters packets according to an aggregate definition. Similarly, the output observation circuit ( 26 ) copies and time stamps the headers of outgoing packets, and filters packets according to the aggregate definition. The incoming and outgoing traces are correlated to calculate a delay measurement. A packet loss measurement uses input observation circuits ( 72   a  and  72   b ) at the input interfaces ( 14 ) of upstream and downstream nodes ( 20  and  22 ) and an output observation circuit  74  at the output interface ( 16 ) of the upstream node. The observation circuits ( 72   a   , 72   b  and  74 ) determine the number of lost packets across a node and between nodes, according to an aggregate definition.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates in general to telecommunications and, moreparticularly, to a method and apparatus to obtain per-hop, one-way,packet loss and delay measurements in multi-class service networks.

2. Description of the Related Art

Per-hop, one-way, packet loss and delay measurements are useful indesigning and maintaining a communications network. Such measurementscan reveal if a piece of equipment is faulty, or not performing tospecifications.

Existing solutions for delay measurements can be categorized as activemeasurement techniques or passive measurement techniques. Solutionsbased on an active measurement technique generate test packets or probesand inject them into the network. Some active measurement solutionsmeasure a roundtrip delay and estimate a one-way delay by halving theresult. Other solutions measure a one-way delay, which is more accurate.In all cases, the one-way active measurement requires a high-precisionclock synchronization in order to determine the delay.

Solutions based on a passive measurement technique traditionally observethe time difference of a packet traversing between two points. Thesetechniques, however, need to resolve a problem of clock synchronizationbetween the two measurement engines, usually located at two differentnodes. Some solutions approach the problem by employing a centralizedsynchonizaton or coordination system.

The major problem with the existing solutions is an ability toaccommodate a large-scale deployment on a per-hop per-aggregate basis.They are either not cost effective or network resource usage effectivein the case of active measurement based solutions. Meanwhile in the caseof passive measurement based solutions, they either introduce excessivecomplexity or employ a centralized control that would restrict a scaleof deployment and be a vulnerable spot.

The prior art has many shortcomings. First, the main problem with theroundtrip delay concerns with a well-known asymmetric route connectingtwo points in a network. Normally, the forward route takes a differencepath from the backward route. This factor challenges a validity ofhalving a roundtrip delay in order to infer to a one-way delay.

Second, the one-way active measurement requires a clock synchronizationbetween two measurement points that imposes a major burden onimplementation and cost. It also impedes the solution from a large-scaledeployment.

Third, the nature of an active measurement itself requires injectingoverhead traffic into the network. Thus to measure a one-way per hop andper aggregate delay at each link in the network is a prohibitive taskand very expensive. This is one of the main reasons that significantlylessen appropriateness in adopting these existing solutions.

Fourth, the major problem that prevents a passive measurement basedapproach from to be a viable and efficient solution is a need of clocksynchronization between two nodes to obtain the delay. Varioustechniques have been proposed such as a centralized coordination or aGPS (Global Positioning System)-based clock synchronization system, butthe usefulness is quite restricted to a small-scale deployment.

A similar problem is present with measuring packet loss. As in the caseof measuring one-way delay, existing solutions can be categorized in twoclasses, active or network-based. Existing active measurement solutionsare based on generating a test packet or probe such as ICMP Echo/Replypackets. Others are based on a one-way active measurement that requiresa clock synchronization system in addition to injecting a test packetinto a network. Network-based solutions use a network management systemto collect information about a packet loss. For example, they employSNMP to gather packet loss statistics from MIB (Management InformationBase) that resides and is populated by the managed node or dedicatedagents. Other may employ RMON (Remote Monitoring) probes or agents tocollect packet loss statistics.

The major problem with the existing solutions is an ability to support alarge-scale deployment on a fine-grained per-hop and per-aggregatebasis. They are either not resource usage efficient or cost effective inthe case of active measurement-based solutions. In the case of networkmanagement-based solutions, they lack a desired fine grained measurementsupport as well as are too complex for a large-scale deployment.

Specific problems with ICMP Echo/Reply packets include: (1)—anintroduction of overhead traffic into a network, (2)—a requirement ofICMP support from a measurement nodes, (3) a lack of per-aggregatemeasurement support, and (4) an inability to measure a packet losswithin a node. The major problem with a one-way active measurement arethat these solutions need a synchronization or coordination between twomeasurement points in addition to imposing overhead traffic onto anetwork. These two drawbacks prevent this class of solutions from alarge-scale deployment. Network-based solutions also have significantproblems: (1) they rely on a node to support MIB in order to collectstatistics; moreover, the current standard MIB does not provideper-aggregate statistics, (2) they offer less flexibility andcontrollability to configure and perform a packet loss measurementdynamically and on a on-demand basis, and (3) they do not support ameasurement of packet loss within a node.

Accordingly, a need exists for an improved technique for measuringper-hop one-way delays and per-hop one-way packet loss in multi-classservice networks.

BRIEF SUMMARY OF THE INVENTION

In the present invention, per-hop, one-way performance characteristicsare measured in a packet-based network by defining an aggregatespecification for specifying a type of packet to be included in ameasurement, generating packet headers with associated timestamps forpackets flowing through a first point and generating packet headers withassociated timestamps for packets flowing through a second point. One ormore performance measurements are calculated responsive to thetimestamps associated with corresponding packet headers according to theaggregate specification.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a reference model for describing terms used incomputing a per-hop one-way delay in a multi-class service network;

FIG. 2 illustrates a block diagram of a preferred embodiment of acircuit for measuring per-hop, one-way, delay between an upstream nodeand a downstream node;

FIG. 3 illustrates a record output from header copier and time stampercircuits;

FIG. 4 illustrates an input trace record for a trace produced by aninput trace generator for each packet in the aggregate;

FIG. 5 illustrates an output trace record for a trace produced by anoutput trace generator for each packet in the aggregate;

FIG. 6 illustrates the structure of a record produced by a correlatorfor each matched pair of incoming and outgoing packets;

FIG. 7 shows a structure for per-hop one-way delay measurement betweenupstream node and downstream node 22

FIG. 8 illustrates an input trace record of a trace generator at aninput interface;

FIG. 9 illustrates a output trace record of a trace generator at theoutput interface of an upstream node;

FIG. 10 illustrates a record output from a first correlator;

FIG. 11 illustrates a record output from a second correlator;

FIG. 12 illustrates a nodal packet loss record; and

FIG. 13 illustrates a line packet loss record.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is best understood in relation to FIGS. 1-13 ofthe drawings, like numerals being used for like elements of the variousdrawings.

FIG. 1 illustrates a reference model for describing terms used incomputing a per-hop one-way delay in a multi-class service network. FIG.1 shows a network 10 with a plurality of network nodes 12 (shownindividually as Node0, Node1, Node2 and Node3). In the illustratedembodiment, data flows sequentially through the nodes, arriving at eachnode's input interface 14 and departing at each node's output interface16.

Looking specifically at Node1, a data packet arrives at the inputinterface of the node 12 at time T₁ and leaves at the output interface16 of the node 12 at time T₂. At time T₃, the packet that was outputfrom Node1 at time T₂ arrives at the input interface 14 of Node2.

The “Per-Hop Reference Section” is defined between two adjacent nodeswith a unidirectional link connecting them. The reference starts fromthe input interface of the upstream node (i.e., point (1)), continuesalong and passes through the upstream node itself to its correspondingoutput interface. Finally, it terminates at the input interface of theadjacent downstream node (i.e., point (3)).

The “Per-Hop One-Way Delay” is defined as a time a packet taking totraverse from starting point of the per-hop reference (point (1)), tothe terminating point of the reference (point (3)). It can be decomposedinto two components the “Per-Hop One-Way Node Delay” and the “Per-HopOne-Way Line Delay”.

The Per-Hop One-Way Node Delay (DelayNode) is the time spent by a packetto pass through the node including at least a queuing time, a processingtime, and a routing and switching time. It can be expressed as,DelayNode=T₂−T₁, where T₁ is the time, a packet is observed at point (1)and T₂ is the time the same packet is observed at point 2.

The Per-Hop One-Way Line Delay (DelayLine) is the time spent by a packetto traverse the associated link. It includes at least a packettransmitting time and a propagation time. Mathematically, it isDelayLine=T₃−T₂, where T₃ is the time the same packet is observed atpoint (3).

The present invention, as it relates to per-hop one-way delay, uses ameasurement based on the difference of time spent by a packet traversingfrom an input interface to the corresponding output interface of thesame node, rather than on a measurement of a time difference spentbetween two nodes locating far apart. This eliminates a need for clocksynchronization since a single clock system, i.e., the node clock, canperform the timing. Further, the measurements can be performed withoutgenerating any additional traffic into a network, using a passivemeasurement approach.

FIG. 2 illustrates a block diagram of a preferred embodiment of acircuit for measuring per-hop, one-way, delay between an upstream node20 and a downstream node 22. Input observation circuitry 24 is coupledto the input interface 14 of the upstream node 20, while outputobservation circuitry 26 is coupled to the output interface 16 ofupstream node 20. Input observation circuitry 24 includes header copier28, time stamper 30, filter 32 and trace generator 34. Outputobservation circuitry 26 includes header copier 36, time stamper 38,filter 40 and trace generator 42. The outputs of trace generators 34 and42 are coupled to correlator 44. Output report generator 46 uses theresults from correlator 44 to generate a report with the measurementdata.

In operation, a passive measurement technique is used to collect packetheader information from the passing-by packets—no artificial packets areinjected into the stream of packets passing through upstream node 20. Aflow definition is provided to specify which packets flowing through theupstream node 20 will be traced.

The header copier 28 of the input observation circuit 24 creates recordsof header information from either all packets or sample packets enteringthe node 20 at the input interface 14. Likewise, the header copier 36 ofthe output observation circuit 26 creates records of header informationfrom either all packets or samples packets exiting the node 20 at theoutput interface 16. Time stampers 30 and 38 (coupled to the node'sclock) add a current time field to each record indicating the time atwhich the packet reached the respective input interface 14 or outputinterface 16. An example record 50 is shown in FIG. 3. The record 50includes a Node Identification field 52, Input/Output Interface ID field54, Time Stamp field 56 and Packet Header 58.

Returning again to FIG. 2, the filters 32 and 40 receive the recordsfrom the respective time stampers 30 and 38. Filters 32 and 40 grouppackets in the defined aggregate, i.e., the packets that conform to theflow definition. The flow definition may include, for example, a DSCP(Differentiated Service Code Point), a source address, a destinationaddress, or any combination of these parameters. Trace generators 32 and40 create a trace of packet header records belonging to the definedaggregate. FIG. 4 illustrates an input trace record 60 for a traceproduced by input trace generator 32 for each packet in the aggregateand FIG. 5 illustrates an output trace record 62 for a trace produced byoutput trace generator 40 for each packet in the aggregate. Input tracerecord 60 includes fields for a node ID, input interface ID, DSCP,source address, destination address, aggregate specification, time stamp(T₁), packet length and packet ID and offset. Output trace record 62includes fields for a node ID, output interface ID, DSCP, sourceaddress, destination address, aggregate specification, time stamp (T₁),packet length and packet ID and offset. Other fields could be includedfor other parameters upon which the trace definition was based.

The correlator 44 matches the traces obtained from the input tracegenerator 34 and the associated output trace generator 42. It makes useof information from the forwarding routing table and configuration fileto associate a packet entering at one input interface 14 to a packetleaving at an output interface 16. This is accomplished by checking thefollowing:

same node ID,

same Src and Dest address,

same DSCP,

same aggregate specification (if multiple tests are performedsimultaneously on a single stream),

same packet ID and the Offset is 0 (for IPv4),

same packet length or payload length,

a valid pair of input and output interfaces, considering from therouting information and the (Src addr, Dest addr).

FIG. 6 illustrates the structure of a record 64 produced by correlator44 for each matched pair of incoming and outgoing packets. Record 64includes fields for node ID, input interface, output interface,aggregate specification and computed delay.

The per-hop one-way delay is defined as a summation of a one-way nodaldelay (Delay_(Node)) and a one-way line delay (Delay_(Line)). A per-hopand per aggregate delay (Delay_(HD)) is defined as the time that apacket of the underlying aggregate spends to traverse a per-hopreference section. Given an output record of underlying aggregate, theper-hop per aggregate nodal delay (Delay_(NHD)) and line delay(Delay_(LHD)) are defined as:Delay_(NHD)=(T ₂ −T ₁)Delay_(LHD)=(Packet Length/Line Speed)+Propagation TimeDelay_(HD)=Delay_(NHD)+Delay_(LHD)

where T₁ is the timestamp of a record produced at the input interface,T₂ is a timestamp of the matched record produced at the output interfaceand packet length is obtained from either of the two records.

This solution allows the most cost-effective, efficient, and scalablemeasurement of a per-hop and per-aggregate one-way delay to alarge-scale deployment in a multi-class service network. First, theinvention provides a per-hop and per-aggregate one-way delay where anaggregate level can be dynamically defined through a packet filterprofile. Second, the invention provides a scalable solution, withoutimposing any overhead traffic onto a network. Third, the invention canbe implemented simply, without high-precision clock synchronization.Fourth, the invention offers an economical and amicable solution for alarge-scale deployment because it does not need a sophisticated clocksynchronization system and particularly it does not generate additionaltraffic into a network. Fifth, the invention provides an accuratesolution, without perturbing a network state during a measurement; thus,an obtained measurement truly reflects an actual network performance.

Referring again to FIG. 1, a second aspect of the invention isdiscussed—a measurement to obtain per-hop one-way packet loss in amulti-class service network. The Per-Hop Reference Section is the sameas described above. “Per-Hop One-Way Packet Loss” is defined a number ofpackets lost during a certain time interval while traversing from thestarting point (1) to the terminating point (3). It can be decomposedinto two parts: Per-Hop One-Way Node Packet Loss (PktLoss_(Node)) andPer-Hop One-Way Line Packet Loss (PktLoss_(Line)). Per-Hop One-Way NodePacket Loss accounts for the number of packets dropped inside of a nodeand can be expressed as PktLoss_(Node)=Count₂−Count₁, where Count₁ isnumber of packets that pass across point (1) and are destined for point(2), in a specified time interval and Count₂ is a portion of Count₁ thatsuccessfully reach point (2). Per-Hop One-Way Line Packet Loss(PktLoss_(Line)) accounts for packets dropped on a link connecting twoadjacent nodes and can be defined as PktLoss_(Line)=Count₃−Count₂.

As in the case of the per-hop one-way delay measurement described inconnection with FIGS. 2-6, measuring the per-hop, one-way packet loss isperformed by defining a flow as an aggregate of packets and employing apassive measurement technique to identify or filter only the packetsthat belong to the flow definition. Packets are observed at three pointsof a reference hop by gathering a trace of packets belonging to the flowat each of the points. Packets in the traces are then correlated todetect any missing packets in order to count a number of unsuccessfullytransferred packets through the hop during a predefined time interval.The packet loss counts are then used to determine a per-hop packet lossof the flow.

FIG. 7 shows a structure 70 for per-hop one-way delay measurementbetween upstream node 20 and downstream node 22. Input observationcircuits 72 a and 72 b are coupled to the input interface 14 a of theupstream node 20 and the input interface 14 b of downstream node 22 andoutput observation circuitry 74 is coupled to the output interface 16 ofupstream node 20. Input observation circuits 72 a and 72 b and outputobservation circuit 74 include header copier 76, time stamper 78, filter80 and trace generator 82. The outputs of the trace generators 82 frominput observation circuits 72 a and output observation circuit 74 arecoupled to a first correlator 84 and output report generator 86. Theoutputs of the trace generators 82 from input observation circuits 72 band output observation circuit 74 are coupled to a second correlator 88and output report generator 90. The measurement data from the outputreport generators 86 and 90 is coupled to analyzer 92.

The header copiers 76 and time stampers 78 operate in the same manner asdiscussed in connection with FIGS. 2 and 3, creating records of headerinformation from all packets entering (for input observation circuits 72a and 72 b) or exiting a node (for output observation circuit 74) andadding a time field to each record. The records produced by headercopiers 74 and time stampers 76 can be the same as shown in FIG. 3.

Likewise, the filters 80 and trace generators 82 operate similarly tothe filters and trace generators described above. Filters 80 performclassification by grouping the packet header records according to anaggregate specification. Trace Generators 82 create a trace of packetheader records belonging to each aggregate. FIG. 8 illustrates an inputtrace record 96 of a trace generator 82 at an input interface (at boththe upstream and downstream nodes) and FIG. 9 illustrates a output tracerecord 98 of a trace generator 82 at the output interface (of anupstream node). An aggregate can be specified by, for example, a DSCP, asource address, a destination address, or a combination of theseparameters.

The correlators 84 and 88 match the traces obtained at the threeobservation points in order to produce a result report. They make use ofthe routing table information in order to associate a packet entering atone input interface to a packet leaving the upstream node outputinterface, as well as to associate a packet traversing from the upstreamnode output interface to the downstream node input interface. Eachcorrelator 84 and 88 associates: (1) the node ID, (2) the interface, (3)source and destination addresses, (4) packet identification (for IPv4)and (5) the packet length or payload length. The output reportgenerators 86 and 90 produce reports of the trace correlating. The firstcorrelator output record 100, output from the first correlator 84, isshown in FIG. 10 and the second correlator output record 102, outputfrom the second correlator 88, is shown in FIG. 11.

The per-hop, per-aggregate packet loss (PktLoss_(HD)) composes of twocomponents: the nodal packet loss and the line packet loss. The per-hop,per-aggregate nodal packet loss (PktLoss_(NHD)) is a loss of packets ofan underlying aggregate that traverse from the input interface to thecorresponding output interface within a node. It may be a result of, forexample, a buffer overflow or an error of packet header. The per-hop,per-aggregate line packet loss (PktLoss_(LHD)) is a loss of packets ofan underlying aggregate on the link connecting two adjacent nodes.Accordingly, PktLoss_(HD)=PktLoss_(NHD)+PktLoss_(LHD).

To compute PktLoss_(NHD) in the first correlator 84 from the tracesoutput by the trace generators of the input observation circuitry 72 aand the output observation circuitry 74. First, a measurement interval,Interval, and a PktLossNode_Time_Threshold are defined. The first recordin the input interface trace that matches the record of the trace fromthe output interface is located by checking for: (1) same node ID, (2)same source and destination addresses, (3) a valid pair of input andoutput interfaces, considering the routing information and the sourceand destination addresses, (4) same DSCP, (5) same aggregatespecification (if multiple simultaneous tests are being performed), (6)same packet ID and offset=0 (for IPv4), and same (7) same packet lengthor payload length. Variables Start_time and Start_time_input are set tothe Time Stamp in the input interface record. Start_time_output is setto the Time Stamp in the counterpart output interface record.

For every following record of the input interface trace with the TimeStamp not exceeding the (Start_time_input+Interval), time_input is setto the corresponding Time Stamp, all similar attributes are checked, asgiven above, to see if there is a counterpart record from the outputinterface trace that has its Time Stamp not exceeding(time_input+PktLossNode_Time_Threshold). If there is no associatedrecord from the output interface trace, the Node_Pkt_Loss counter isincremented. If there is an associated record with Time Stamp greaterthan (time_input+PktLossNode_Time_Threshold), then the Node_Pkt_Losscounter is incremented as well as deletion of the entry from the trace.When the Time Stamp of the input interface record is greater than the(Start_time_input+Interval), Stop_time is set to (Start_time+Interval)and the nodal packet loss record 104 shown in FIG. 12 is generated.

PktLoss_(LHD) is computed using a similar method. Start_time, Interval,and Stop_time are set as above, PktLossLine_Time_Threshold is defined.The record from the input interface trace of the downstream node thatmatches the first record from the output interface of the upstream nodeis located by checking all the followings attributes: (1) a valid pairof upstream and downstream node IDs, (2) same source and destinationaddresses, (3) same DSCP, (4) same aggregate specification (if multiplesimultaneous tests are being performed), (5) same packet ID and theOffset is 0 (for IPv4), (6) same packet length or payload length, (7) avalid pair of input and output interfaces, considering from the routinginformation and the source and destination addresses.Start_time_upstream is set to the Time Stamp of the first outputinterface record of the upstream node and Start_time_downstream is setto the Time Stamp in the obtained counterpart input interface record ofthe downstream node.

For every following record of the output interface trace of the upstreamnode successfully obtained, all similar attributes are checked, as givenabove, to see if there is a counterpart record from the input interfacetrace of the downstream node that has the Time Stamp not exceeding(Start_time_downstream+PktLossLine_Time_Threshold). If there is noassociated record from the output interface trace, the Line_PktLosscounter is incremented. If there is an associated record with Timestampgreater than (Start_time_downstream+PktLossLine_Time_Threshold), thenthe Line_PktLoss counter is incremented. After the last record from theoutput interface trace of the upstream node is checked, the line packetloss record 106 shown in FIG. 13 is generated.

The analyzer 92 uses the nodal packet loss records 104 and line packetloss records 106 to determine a per-hop, per aggregate, one-way, packetloss. The analyzer 92 matches the attributes from these records, asshown in Table 1.

TABLE 1 Record Matching by Analyzer Nodal Packet Loss Record Line PacketLoss Record Node ID Upstream Node ID Output Interface Output InterfaceAggregate Specification Aggregate Specification Start Time Start TimeStop Time Stop Time Interval Interval

For a pair of matched records, the PktLoss_(HD) is computed from thesummation of the values in the packet loss count fields of the records.

This solution allows the most cost-effective and scalable measurement ofa per-hop and per-aggregate one-way packet loss for a network-widedeployment in a multi-class services network and provides manyadvantages over existing solutions. First, it provides a per-hop andper-aggregate one-way packet loss where an aggregate level can bedynamically defined through a packet filter profile. Second, itaccommodates a wide range of aggregate level from per-source ordestination IP address, network service class, or DSCP/PHB, to per ATMVPI/VCI or LSP. Third, it also accommodates an aggregation hierarchy.Since an aggregation is specified through a filter profile, it is thusflexible in changing an aggregate level to meet the on-demandrequirement. Fourth, it provides a control on a temporal granularity ofan obtained result. By explicitly specifying a collecting interval andtime (interval time), various time scale results can be obtained for anytime periods. Fifth, it provides an accurate result due to a passivemeasurement and does not inject additional traffic that may perturb thenetwork. Sixth, it provides scalability of deployment due to a passivemeasurement technique. Seventh, it offers a cost-effective and simplesolution for a large-scale deployment and does not require a complexclock or coordination system.

The embodiment of FIG. 7 could be used for both packet loss and delaymeasurements. The only necessary change would be to calculate the delaybetween input interface 14 a and output interface 16 of the upstreamnode 20 for each of the filtered packets in the first correlator 84, andadd this information to record 104 in order to pass this information tothe analyzer 92.

It should be noted that in the structures of FIGS. 2 and 7, timestamping occurs prior to filtering. While filtering according to theaggregate specification could be performed prior to copying the headerand time stamping, the time delay involved in filtering (and,specifically, the variation in the time delay in filtering at thedifferent checkpoints) could reduce the accuracy of the time stamping.

Although the Detailed Description of the invention has been directed tocertain exemplary embodiments, various modifications of theseembodiments, as well as alternative embodiments, will be suggested tothose skilled in the art. The invention encompasses any modifications oralternative embodiments that fall within the scope of the claims.

1. A method of measuring per-hop, one-way performance characteristics ina packet-based network, comprising the steps of: defining an aggregatespecification for specifying a type of packet to be included in ameasurement; generating packet headers with associated timestamps forpackets flowing through a first point; generating packet headers withassociated timestamps for packets flowing through a second point;generating packet headers with associated timestamps for packets flowingthrough a third point; and calculating one or more performancemeasurements responsive to the timestamps associated with correspondingpacket headers according to the aggregate specification.
 2. The methodof claim 1 and further comprising the step of filtering packet headersresponsive to the aggregate specification.
 3. The method of claim 1wherein said first point is the input interface of a node and the secondpoint is the output interface of the node and wherein said calculatingstep comprises the step of calculating a delay measurement.
 4. Themethod of claim 1 and further comprising the step of filtering packetheaders at said first, second and third points according to theaggregate specification.
 5. The method of claim 1 wherein said firstpoint is the input interface of a first node, the second point is theoutput interface of the first node, and the third point is the inputinterface of a second node and wherein said calculating step comprisesthe step of calculating a packet loss measurement.
 6. Circuitry formeasuring per-hop, one-way performance characteristics in a packet-basednetwork, comprising: circuitry for generating packet headers withassociated timestamps for packets flowing through a first point;circuitry for generating packet headers with associated timestamps forpackets flowing through a second point; circuitry for generating packetheaders with associated timestamps for packets flowing through a thirdpoint; and circuitry for calculating one or more performancemeasurements responsive to the timestamps associated with correspondingpacket headers according to a predefined aggregate specification.
 7. Thecircuit of claim 6 and further comprising circuitry for filtering packetheaders responsive to the aggregate specification.
 8. The circuit ofclaim 6 wherein said first point is the input interface of a node andthe second point is the output interface of the node and wherein saidcalculating circuitry comprises circuitry for calculating a delaymeasurement.
 9. The circuit of claim 6 and further comprising circuitryfor filtering packet headers at said first, second and third pointsaccording to the aggregate specification.
 10. The circuit of claim 6wherein said first point is the input interface of a first node, thesecond point is the output interface of the first node, and the thirdpoint is the input interface of a second node and wherein saidcalculating circuitry comprises circuitry for calculating a packet lossmeasurement.